According to records of 338 earthquakes in recent years by SK seismograph of Tianshui seismic station,a formula of relationship between travel-time difference of maximum surface wave from primary wave and epicentral distance is given by least squares method.It is discovered there is much difference between epicentral distance calculated by using the J-B travel-time table and that of practical observation,the difference is small by using the formula and travel-time table of this paper by comparison.